Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Among the types of desired information is the extent and distribution of fluids in the reservoir formations. While it is possible to glean a general picture with surface surveys, such surveys are limited by the effects of the subsurface layers overlying the region of interest. Such effects can be eliminated or reduced by the use of boreholes in or near the region of interest. With a suitable arrangement of borehole transmitters and receivers, crosswell tomography can be used to extract a comparatively detailed image of the region of interest, suitable for planning and monitoring production from a reservoir.
Initially, crosswell tomography was performed using seismic transmitters and receivers, but more recently the focus has been on the use of electromagnetic (EM) transmitters and receivers. As with any geophysical survey, noise and inaccuracies in the survey system will negatively impact image quality. One important cause of this degradation in crosswell tomography is mismatches between assumed and actual values for sensor gains and phases. These mismatches can result from a number of causes including differences in sensor types, calibration errors, variation in borehole configurations (e.g., mud composition and borehole radius), and environmental parameters such as temperature and pressure. Phase mismatches can be caused or exacerbated by latencies in electronic systems and synchronization errors for the widely-distributed components of a typical crosswell tomography system. Traditional methods crosswell tomography do not appear to adequately address such inaccuracies.
While the invention is susceptible to various alternative forms, equivalents, and modifications, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the described embodiments by the scope of the appended claims.